Dyes known as oxidation dyes, which are produced by oxidative coupling of one or more developer components with one or more coupling components, are preferred due to their intense colors, the mild reaction conditions under which they are formed, and their very good fastness properties. Nitrogen bases such as primary aromatic amines with an additional hydroxyl or unsubstituted or substituted amino group in ortho- or para-position, diamino-pyridine derivatives, 4-amino-pyrazolone derivatives, heterocyclic hydrazone derivatives, and tetraaminopyrimidines are generally used as developer substances. Phenols, m-phenylene diamine derivatives, naphthols, certain resorcinol derivatives, and pyrazolones are known to be useful as coupling components.
Good oxidation dyestuff components must meet the following requirements: They must produce the desired color nuances in sufficient intensity during oxidative coupling with the respective developer or coupling component. Also, they must possess a capacity for being absorbed by human hair without excessive coloring of the scalp. In addition, they should be toxicologically and dermatologically safe.
The production of the strongest possible color shades closely corresponding to the natural hair color nuances is also important. Furthermore, the general stability of the dyestuffs produced as well as their fastness to light and to washing and their thermostability, have very special significance for the prevention of color shifts from the original color nuance or even a change in color to different shades. In addition, in the hair dyeing field there is always an interest in new oxidation dye components that can be combined with the known dye components to produce new color nuances of cosmetic value.
The use of diaminopyridines as oxidation dye precursors is known from German Offenlegungsschrift (DE-OS) No. 11 42 045. German Offenlegungsschrift (DE-OS) No. 14 92 158 describes other amino- and oxy-pyridines for the production of oxidation dyes. However, the oxidation dyes obtained from the pyridine derivatives described in those publications are unsatisfactory with regard to both depth of color and fastness.
German Offenlegungsschrift (DE-OS) No. 16 17 831 describes a process for dyeing human hair using pyridine dye precursors, in which a quantity of reducing agent exceeding the concentration normally used to prevent the premature oxidation of dye precursors is added to obtain greater depth of color. This process has the disadvantage that benzoic dye precursors, for example, couplers, cannot be used for modifying the colors because they are excessively weakened by the reducing agent used.
Accordingly, the search for better oxidation hair dyes includes the task of finding and combining with one another suitable oxidation dye precursors which give both strong and deep colors and which also show high stability to heat and light. In addition, suitable oxidation dye precursors must demonstrate favorable toxicological and dermatological properties.